1. Field
Apparatuses and methods consistent with exemplary embodiments relate to an optical device, and more particularly, to a large-area transmissive type optical image modulator and a method of manufacturing the same, and an optical apparatus including the transmissive type optical image modulator.
2. Description of the Related Art
In order to obtain information regarding a range image, a stereo vision method using two cameras and a triangulation method using structured light and a camera are used.
However, when these methods are used, as the distance between the camera and the object is increased, precision may rapidly decrease. Also, these methods are dependent on a surface state of an object, and thus it may be difficult to obtain a minute range image.
In order to solve the above-described problem, a time-of-flight (TOF) method has been introduced. The TOF method is used to measure flight time in which separate modulated light, for example, a laser or light emitted from a light-emitting diode (LED), is radiated onto an object and then is returned to a light-receiving unit (a sensor or charged-coupled device (CCD) camera).
One TOF method is classified as a direct time measuring method of projecting pulse light and measuring time in which the pulse light is returned and a correlation method of measuring a distance from brightness information obtained by multiplying a gating signal synchronized with emitted light in a light receiving unit by a reflected light signal. A brightness image obtained by the correlation is bright at a short distance because of a large amount of light received and is dark at a long distance because of a small amount of light received.
Another TOF method includes a phase delay measuring method in which sine wave light or continuous wave (CW) light is projected and a phase difference of reflected light returning is sensed to determine a distance.
The phase delay measuring method is classified as an external modulation method or an internal modulation method.
In the external modulation method, a phase difference is measured by a CCD camera by projecting a sine wave that is frequency-modulated in an LED or a laser diode (LD) and modulating (mixing) an amplitude by a light modulator located at a front end of a light receiving unit to be converted into a low-frequency signal.
In the internal modulation method, a phase delay is measured by amplitude modulation or demodulation inside a CCD cell without an optical modulator. In order to perform modulation and demodulation inside a CCD cell, an additional function of processing signals around the CCD is required, and thus a fill factor deteriorates and a size of the CCD cell is increased, thereby imposing limitations in forming a high resolution distance camera.
A transmissive optical modulator includes an optical modulator using a Pockel effect or a Kerr effect based on an image intensifier or crystal optics. Such an optical modulator is bulky, uses a high voltage of several KV, and is expensive.
Another transmissive optical modulator includes a transmissive optical image shutter using GaAs-based electro-absorption. This transmissive optical modulator has a PIN diode structure in which an intrinsic layer between PN electrodes includes a multiple quantum well and forms excitons in a specific wavelength area to facilitate electro-absorption. If a bias voltage is applied to both ends of the PN electrode in a reverse direction, a maximum value of an absorption coefficient is decreased according to wavelengths and is red-shifted. If a peak of an absorption wavelength is in accordance with a Fabry-Perot wavelength, maximum optical absorption occurs, and brightness of light to be output may be controlled according to voltages. In such a transmissive optical modulator, an opaque GaAs substrate may be removed for light transmission.
After the GaAs substrate is removed, a remaining structure may be transferred to a transparent SiO2 substrate. However, a series of processes, in which a substrate of an expitaxial structure in which an electrode is formed during a process of manufacturing a wafer level is removed and transferred to another substrate, that is, an SiO2 substrate, may be complex, and thus it may be difficult to stably perform the processes.
Recently, although a transmissive optical modulator, in which a portion through which light passes is removed from a conventional opaque substrate and a transparent InGaP layer is added to an expitaxial layer to be used as a support for supporting an expitaxial structure, has been introduced, an expitaxial thin film may be vulnerable to an external impact or a mechanical transformation.